1. Field of the Invention
The present invention relates to an image forming apparatus, such as a copy machine, and a fixing device included therein, and more particularly to a fixing device for fixing a toner image on a transfer target in a manner based on an induction heating technique, and an image forming apparatus using the fixing device.
2. Description of the Related Art
An image forming apparatus is designed to irradiate an outer peripheral surface of a photosensitive drum in a rotating state with an image information-based light beam so as to form an electrostatic latent image on the outer peripheral surface, and supply toner serving as developer to the latent image so as to a toner image. The toner image formed on the outer peripheral surface of the photosensitive drum is transferred onto a sheet serving as a transfer target fed thereto, and then the sheet is subjected to a fixing process based on heating in a fixing device. The sheet after completion of the fixing process is ejected outside from an apparatus body.
Typically, the fixing device comprises a fixing roller adapted to be heated to a high temperature, and a pressing roller disposed opposed to the fixing roller in such a manner that an outer peripheral surface thereof is in contact with an outer peripheral surface of the fixing roller. The fixing process is performed by feeding a sheet into a nip zone defined between the fixing and pressing rollers. Heretofore, a built-in type halogen lamp has been employed as a heating source for the fixing roller. The halogen lamp has problems about poor thermal efficiency, and slow response (or low heat-up speed) requiring a fairly long time-period in a warming-up (initial heating) stage. While various techniques for achieving reduction in heat capacity and wall thickness of the fixing roller have been developed as measures against these problems, such approaches have limitations for themselves.
Recent years, great interest has been shown in an induction heating-type fixing device designed to heat a fixing roller based on an induction heating technique, as disclosed in Japanese Patent Laid-Open Publication No. 09-127810. In this induction heating-type fixing device, the fixing roller comprises a hollow roller made of a nonmagnetic metal having excellent heat conductivity, and a thin layer formed on an outer peripheral surface of the hollow metal roller and made of a magnetic metal. The fixing device is provided with an induction coil within the fixing roller, and designed to energize the induction coil so as to produce an eddy current in the magnetic metal layer and heat the fixing roller based on Joule heat generated by the eddy current.
As compared with the conventional halogen lamp-type fixing device, the induction heating-type fixing device allows the fixing roller to be heated up at a drastically increased speed so as to achieve a higher-speed warm-up of the fixing roller. On the other hand, the extremely high heat-up speed raises a new problem about excessive heating of the fixing roller. In order to solve this problem, it is contemplated to employ a feedback control for detecting a temperature of the fixing roller using a temperature sensor, such as a thermistor or a thermostat, and cutting off a power supply to the induction coil when the fixing roller is heated up to a predetermined temperature or more. However, the temperature sensor has difficulty in outputting a detection signal accurately in response to a temperature rise arising from the induction heating, and this time-lag or detection delay is likely to preclude prevention of excessive heating of the fixing roller.
Generally, heat transfer in a longitudinal direction of a fixing roller is apt to become harder as the fixing roller is reduced in wall thickness. Thus, when a sheet having a width less than a heating width of the fixing roller is continuously passed through the fixing roller (or a nip zone), heat tends to stay and accumulate at the opposite end regions of the fixing roller that a smaller number of sheets pass. In this state, if wider sheets are subjected to a fixing process, the accumulated heat will cause image defects, such as a so-called offset phenomenon that a toner image on one of the wider sheets is fusion-bonded onto the end regions of the fixing roller and then transferred onto the next wider sheet.
In order to solve this problem, Japanese Patent Laid-Open Publication No. 2004-151470 (hereinafter referred to as Document D2) discloses an induction heating-type fixing device comprising a fixing roller which includes a tubular-shaped temperature-sensitive metal layer made of a temperature compensator alloy, a nonmagnetic metal layer formed on an outer peripheral surface of the temperature-sensitive metal layer in a concentric manner, and an induction coil disposed inside the tubular-shaped temperature-sensitive metal layer and adapted to generate a magnetic field. In this fixing roller, the temperature-sensitive metal layer has a thickness t (m) set to satisfy the following inequality:
            503      ×                        ρ          /                      (                          μ              ⁢                                                          ⁢              s              ×              f                        )                                <    t    <          503      ×                        ρ          /                      (                          1              ×              f                        )                                ⁢          ,
wherein: ρ is a resistivity of the magnetic shunt alloy (Ω·m); f is a frequency (Hz) of a power supply for the induction coil; and μs is a relative permeability of the magnetic shunt alloy at a temperature less than a Curie temperature thereof.
In the above inequality,
  503  ×            ρ      /              (                  μ          ⁢                                          ⁢          s          ×          f                )            is a magnetic-field penetration depth when the temperature-sensitive metal layer has a temperature less than the Curie temperature (transition temperature), and
  503  ×            ρ      /              (                  1          ×          f                )            is a magnetic-field penetration depth when the temperature-sensitive metal layer has a temperature equal to or greater than the Curie temperature.
In this fixing roller, when the temperature-sensitive metal layer has a temperature less than the Curie temperature, a magnetic-field penetration depth becomes less than the thickness of the temperature-sensitive metal layer. Thus, a load (electric resistance) to an eddy current generated by the magnetic field is increased (i.e., an eddy current flows through a narrow area at higher density and a load to the eddy current is increased), and thereby a magnetic flux flows through the temperature-sensitive metal layer with a large electric resistance in an axial direction thereof. The increased load to the eddy current will generate a larger quantity of heat (Joule heat) to allow the temperature-sensitive metal layer to be quickly heated up.
Then, when the temperature-sensitive metal layer is heated up to a temperature equal to or greater than the Curie temperature, a magnetic-field penetration depth becomes greater than the thickness of the temperature-sensitive metal layer. Thus, the magnetic field reaches the nonmagnetic metal layer with a lower resistivity than that of the temperature-sensitive metal layer, and a magnetic flux flows through the low-resistivity nonmagnetic metal layer in the axial direction. This makes it possible to reduce a heat generation rate and suppress excess heating of the fixing roller.
As above, this fixing roller has an effect of being able to prevent excess heating thereof without using the aforementioned control intended to suppress excess heating of a fixing roller based on detection of a temperature of the fixing roller using a temperature sensor, such as a thermistor or a thermostat (i.e., without the risk of occurrence of control lag due to output delay of a detection signal).
Just for reference, in the fixing roller disclosed in the Document D2, an alloy of iron (Fe) and nickel (Ni) is used as a material as the temperature-sensitive metal layer and formed to have a thickness of 0.3 mm, and aluminum (Al) is used as a material of the nonmagnetic metal layer and formed to have a thickness of 0.7 mm.
In a fixing roller formed with the temperature-sensitive metal layer and the nonmagnetic metal layer as disclosed in the Document D2 wherein materials and dimensions of the fixing roller are selected to satisfy the above inequality, generation of Joule heat can be reduced at a lower level, because, when a temperature of the temperature-sensitive metal layer becomes equal to or greater than its Curie temperature according to excitation of the induction coil for a fixing process, a magnetic field penetrates through the temperature-sensitive metal layer, and a magnetic flux flows across the nonmagnetic metal layer in an axial direction thereof. However, in view of meeting the need for reducing a warming-up time, a metal layer of a fixing roller is required to be further reduced in wall thickness.
If the nonmagnetic metal layer is reduced in thickness without reasonable limit, a load will be increased (i.e. an eddy-current density will be increased) due to reduced eddy-current generation area, to cause difficulty in suppressing generation of Joule heat even when a magnetic field flows through the nonmagnetic metal layer after the temperature-sensitive metal layer becomes equal to or greater than a Curie temperature. As a result, if a fixing process is continuously performed, even the induction heating-type fixing device disclosed in the Document D2 will be excessively heated to cause a problem about excessive temperature rise in opposite end regions of the fixing roller or a region except for a central region thereof where heat is released to sheets passing therethrough.